FIG. 6 is a schematic diagram illustrating a prior art MOCVD apparatus disclosed in "Metal Organic Vapor Phase Epitaxy of Indium Phosphide", Journal of Crystal Growth, 64 (1983), pp. 68-75. In FIG. 6, reference numeral 1 designates a reactor. A graphite susceptor 2 is contained in the reactor 1 and heated by a high-frequency induction heating coil 5 surrounding the reactor 1. A wafer 3 is disposed on the susceptor 2. Reference numeral 4 designates a source gas flow.
Source gases introduced into the reactor 1, such as trimethylindium (TMI), trimethylgallium (TMG), phosphine (PH.sub.3), and arsine (AsH.sub.3), are thermally decomposed in the presence of the induction heated susceptor 2 and elements of those gases are deposited on the wafer 3 on the susceptor 2.
FIG. 7 is a graph for explaining a distribution of composition in an InGaAsP crystal grown on a 3 inch diameter substrate. In FIG. 7, the ordinate shows the peak wavelength of photoluminescence which corresponds to the inverse of the band gap energy of the InGaAsP crystal, and the abscissa shows the mismatch rate of the lattice constant of the InGaAsP crystal to the lattice constant of the InP substrate. For example, when the lattice constant of the InP substrate is "a0" and the lattice constant of the InGaAsP crystal grown thereon is "a", the mismatch is represented by ##EQU1## In the graph, five points are plotted at intervals of 15 mm on the 3 inch diameter wafer from the upstream part to the downstream part of the gas flow. The five points are in a straight line on the graph. The gradient of the plot is the same as that attained when the growth is carried out while varying the flow rates of the V-group source gases, i.e., AsH.sub.3 and PH.sub.3. It is supposed, from this fact, that there is a difference in the incorporation ratios of the V-group atoms, i.e., As atoms and P atoms, in the wafer in the upstream and downstream parts of the source gas flow 4.
The cause of this phenomenon is as follows. The source gas 4 introduced into the reactor is heated on the susceptor 2 and deposited by the wafer 3. However, the temperature of the source gas at the upstream part of the gas flow does not reach a prescribed temperature (growth temperature) but the temperature gradually increases and approaches to the growth temperature while the source gas flows above the susceptor 2. Therefore, when the crystal growth process employs source gases whose decomposition ratios significantly vary depending on the temperature change in the vicinity of the growth temperature, the composition of the crystal grown on the wafer varies significantly from the upstream part to the downstream part of the source gas flow.
The above-described temperature gradient of the source gas 4 on the wafer 3 can be supposed from the doping efficiency of n type or p type impurities because the doping efficiency decreases with an increase in the temperature. That is, the doping efficiency at the downstream part is lower than that at the upper stream part of the gas flow.
In the conventional MOCVD apparatus, the temperature gradient of the source gas adversely affects the uniformity of the crystal composition in the wafer, so that the crystal composition of an end portion of the wafer positioned at the upstream part of the gas flow deviates from the standard value, reducing the yield in the crystal growth process.
Meanwhile, Japanese Published Utility Model Application No. 3-116028 discloses a CVD apparatus including a high frequency coil that is movable in the longitudinal direction of the reactor, whereby a region of a high heating efficiency produced by the high frequency coil is moved from one end to the opposite end of the wafer, reducing the variation in the temperature on the wafer. However, since the temperature gradient of the source gas flow is not reduced by the movable coil, the above-described problem remains unsolved. In this CVD apparatus, the surface of the susceptor, on which the wafer is put, is tilted to increase the gas velocity and decrease the uneveness of the thickness of the boundary layer due to the entrance effect, resulting in a uniform deposition speed over the wafer.
On the other hand, Japanese Published Patent Application No. 63-89967 discloses a CVD apparatus including resistance heating means and a high frequency induction coil which are divided into a plurality of parts in the longitudinal direction of the reactor. Since this CVD apparatus is for making the thickness of a single crystal layer grown on the wafer uniform, the above-described problem remains unsolved. More specifically, in this CVD apparatus, a temperature gradient, that provides a higher temperature at the downstream part of the gas flow and a lower temperature at the upstream part of the gas flow, is generated in the reactor to decrease the decomposition rate of the source gas at the upstream part and increase the deposition speed at the downstream part, resulting in a grown layer of having an even thickness. Therefore, the high frequency induction coil divided into a plurality of parts is used as auxiliary heating means for rapidly adjusting the temperature inside the reactor to the decomposition temperature of the source gas introduced into the reactor when the source gas is changed during the growth process. If a compound crystal is grown using this apparatus, the temperature gradient of the source gas on the wafer is further increased. Therefore, this CVD apparatus does not improve the composition distribution in the compound crystal grown on the wafer.